System and method for communicating timing to a remote node

ABSTRACT

A system and method for synchronizing a clock for data transmissions. A data packet is received at a remote node. A timing characteristic of the data packet corresponds to a tick of a clock form a reference clock. A tick of the clock is determined based on the timing characteristic of the data packet. A secondary clock is disciplined with the reference clock by adjusting the secondary clock based on a difference between times measured by the reference clock and the secondary clock to generate a clock signal. The clock signal is communicated to one or more interfaces.

PRIORITY

This application claims priority to provisional application Ser. No.61/040,474, filed on Mar. 28, 2008, which is incorporated herein byreference.

BACKGROUND

The Internet Protocol (IP) includes the rules and encodingspecifications for sending data. Layer 3 of the open systemsinterconnect (OSI) model is the network layer in which protocols such asIP operate. Layer 3 and above services are generally deliverable viaEthernet physical connections. To meet user expectations for highquality Ethernet services, such as differential mode pseudo-wire, amethod of monitoring packet performance, such as one-way latency, isrequired. One-way latency and other performance measurements require aclock reference with a high degree of accuracy.

Communications between packet networks and time domain multiplexing(TDM) or synchronous systems may further complicate performancetracking. Many existing systems and interfaces, such as Ethernetswitches, do not provide a cost efficient, robust, and reliable systemfor maintaining a clock or timing signal.

SUMMARY

A system and method for synchronizing a clock for data transmissions. Adata packet may be received at a remote node. A timing characteristic ofthe data packet may correspond to a tick of a clock from a referenceclock. A tick of the clock may be determined based on the timingcharacteristic of the data packet. A secondary clock may be disciplinedwith the reference clock by adjusting the secondary clock based on adifference between times measured by the reference clock and thesecondary clock to generate a clock signal. The clock signal may becommunicated to one or more interfaces.

Another embodiment provides a remote node for communication with asynchronous network. The remote node may include an optical interfacefor receiving a data signal through a fiber optic. The interface may beoperable to extract a tick of a reference clock associated with a timingcharacteristic of a data packet communicated through the fiber optic.The remote node may also include a clock in communication with theoptical interface. The interface may discipline the clock utilizing thetick of the reference clock extracted from the timing characteristic ofthe data packet to maintain a clock signal. The remote node may alsoinclude a timing bus in communication with the clock. The timing bus maybe operable to transmit the clock signal to one or more cards. Theremote node may also include one or more interfaces in communicationwith the timing bus. The one or more interfaces may be operable toutilize the clock signal for synchronous communications.

Yet another embodiment provides a system and method for synchronizing aclock for data transmissions. A propagation delay between a sending nodeand a remote node may be determined. A data packet may be received atthe remote node. A leading edge of the packet may correspond to a tickof a clock from a reference clock. The tick of the clock may bedetermined from the leading edge of the data packet. A secondary clockmay be disciplined with the reference clock by adjusting the secondaryclock based on a difference between times measured by the referenceclock and the secondary clock from the tick of the clock and thepropagation delay to generate a clock signal. The clock signal may becommunicated to one or more interfaces.

A system and method for synchronizing a clock signal. Data traffic maybe received through a first channel of a fiber optic. A clock signal maybe received through a second channel of the fiber optic. A clock at anode may be disciplined with the clock signal. The clock signal may besent from the clock to one or more interfaces within a node.

Another embodiment includes a remote node for communicating a clocksignal. The remote node may include an optical interface for receivingdata traffic through a first wavelength of a fiber optic and a clocksignal through a second wavelength of the fiber optic. The remote nodemay also include a clock in communication with the optical interface.The clock may be operable to receive the clock signal. The clock may bedisciplined with the clock signal to maintain the clock signal. Theremote node may include a timing bus in communication with the clock.The timing bus may be operable to transmit the clock signal to one ormore cards. The remote node may also include one or more cards incommunication with the timing bus. The one or more cards may be operableto utilize the clock signal for synchronous communications.

Yet another embodiment includes a metro Ethernet switch. The switch mayinclude an optical interface for receiving communications through aplurality of fiber optics. A first port of the optical interface mayreceive data traffic through a first wavelength of a fiber optic and aclock signal through a second wavelength of the fiber optic. The opticalinterface may include a second port operable to receive the clock signalthrough another fiber optic in response to determining the clock signalis not received through the second wavelength. The switch may alsoinclude a clock in communication with the optical interface. The clockmay be operable to receive the clock signal. The clock may bedisciplined with the clock signal to maintain the clock signal. Theswitch may also include a timing bus in communication with the clock.The timing bus may be operable to transmit the clock signal to one ormore clocks. The switch may also include one or more clocks incommunication with the timing bus. The one or more clocks may beoperable to utilize the clock signal for synchronous communicationsthrough one or more cards.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention are described indetail below with reference to the attached drawing figures, which areincorporated by reference herein and wherein:

FIG. 1 is a pictorial representation of an Ethernet ring in accordancewith an illustrative embodiment;

FIG. 2 is a pictorial representation of a communications environment inaccordance with an illustrative embodiment;

FIG. 3 is a pictorial representation of communications between a centraloffice and a remote node in accordance with an illustrative embodiment.

FIG. 4 is a block diagram of a remote node in accordance with anillustrative embodiment;

FIG. 5 is a flowchart of a process for disciplining a clock of a remotenode in accordance with an illustrative embodiment; and

FIG. 6 is a flowchart of a process for communicating a clock signal inaccordance with an illustrative embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

An illustrative embodiment provides a system, method, and remote nodefor synchronizing a clock of a remote node with a universal timecoordinated (UTC) clock for communications signals. The frequency ortick of a clock may be extracted from a timing characteristic of a datapacket received at a remote node. The timing characteristic is aparameter, indicator, or characteristic of an arriving packet. In oneembodiment, the timing characteristic is a leading or trailing edge of adata packet. Other timing characteristics may also be utilized that mayutilize other information, such as average data packet arrival time orthe center points of data packets.

The clock tick is a designated time unit. The clock tick may representseconds, deciseconds, centiseconds, milliseconds, microseconds,nanoseconds, picoseconds, femtoseconds, or another designatedsubdivision, time, frequency, or multiple of any of the foregoing. Forexample, one hundred or one thousand clock ticks may occur each second.The tick or frequency of the clock may be utilized to discipline orsynchronize the clock output signal of a clock integrated within theremote node. The synchronized clock signal may be communicated to amultiple interfaces which may include cards, ports, subservient clocksand downstream devices, components, and elements for utilization inmultiple communications signals.

Another embodiment provides a system and method for communicating aclock signal from a first location to a second location. In oneembodiment, data traffic is communicated from a central office to aremote node utilizing a first wavelength. A second wavelength may beutilized to propagate a clock signal from a UTC clock or other highlyaccurate clock at the central office to one or more remote nodes. Thedata traffic and clock signal may be propagated through a single port ofan optical Ethernet interface.

FIG. 1 is a pictorial representation of an Ethernet ring 100 inaccordance with an illustrative embodiment. The Ethernet ring 100 is anEthernet topology. In one embodiment, the Ethernet ring may a MetroEthernet network. A metro Ethernet is a layer 2 network that is based onthe Ethernet standard and that covers a metropolitan area. MetroEthernet may be utilized as a metropolitan access network to connectsubscribers and businesses to a Wide Area Network (WAN), such as theInternet.

One or more large businesses, organizations, or other groups may alsouse Metro Ethernet to connect branch offices to an Intranet. TheEthernet ring 100 may be used because Ethernet supports high bandwidthconnections and may be easily integrated with and connect to corporateand residential customer devices, networks, or other resources. In oneembodiment, the Ethernet ring 100 may provide a next generationreplacement of synchronous optical network (SONET) rings.

In one embodiment, the Ethernet ring 100 includes switches 102, 104,106, 108, and 110, and fiber optics 112. The Ethernet ring 100 may alsobe any topology, structure, or design suitable for communication, suchas hub-and-spoke, star, point-to-point, full mesh, partial mesh, orother Ethernet architectures. The Ethernet ring 100 may include anynumber of devices, elements, and connections. For example, in order tocommunicate data through the fiber optics 112, any number of routers,splices, amplifiers, media converters, modulators, multiplexers, lightgenerators, and other elements may be used.

The Ethernet ring 100 is part of a communications network that mayutilize a hierarchy, including a core, distribution, and access. Thecore may be the backbone of the communications network for communicatingsignals, data, and information from one point to another. The switches102, 104, 106, 108, and 110 or other data elements are central points ofconnections for computers and other communications equipment in acommunications network. The switches 102, 104, 106, 108, and 110 may belocated at one or more central offices, nodes, multi-tenant buildings,hardened cabinets, or other service provider or customer facilities.

The fiber optics 112 are thin strands of specially manufactured plasticor glass used to transport and direct light communications from a sourceto a given destination. The communications signals may be generated by alaser, light emitting diode (LED), or other optoelectronic device. Oneor more strands of the fiber optics may transmit and receivecommunications signals between the switches 102, 104, 106, 108, and 110.

The switches 102, 104, 106, 108, and 110 may need to communicate a clocksignal around the Ethernet ring 100 or to an edge aggregator. The edgeaggregator is the point, connection, or device at which access legs incommunication with customer or user equipment interconnect with theEthernet ring 100. Clock timing may be particularly important for TDMemulation over packet communications, or other circuit based or timeslot based communications protocols and standards over packetcommunications. Synchronous TDM includes T1, SONET/SDH, and ISDN.Additionally, a clock signal may be required to perform TDM over anon-synchronous packet network with such emulation modes as differentialmode pseudo wire. In computer networking and telecommunications, apseudo wire is an emulation of a native service over a packet switchednetwork. The native service may be ATM, frame relay, Ethernet, low-rateor high-rate TDM, or SONET/SDH.

FIG. 2 is a pictorial representation of a communications environment 200in accordance with an illustrative embodiment. The communicationsenvironment 200 may include any number of networks, systems, devices,and connections. The communications environment 200 is shown forillustration purposes. In one embodiment, the communications environment200 may include a central office 202, a Stratum clock 204, TDM networks206 and 207, an IP network 208, a remote node 210, a switch 212,customer 213, and a telephone 214. The communications environment 100may include various components, nodes, connections, and devices of theEthernet ring 100 of FIG. 1.

The different elements and components of the communications environment200 may communicate using hardwired connections, such as fiber optics,T1, cable, DSL, high speed trunks, and telephone lines. Alternatively,portions of the communications environment 200 may include wirelesscommunications, including satellite connections, time division multipleaccess (TDMA), code division multiple access (CDMA), global systems formobile (GSM) communications, personal communications systems (PCS),WLAN, WiMAX, or other frequently used cellular and data communicationsprotocols and standards.

The TDM network 206 and 207, and IP network 208 may communicate with anynumber of networks which may include wireless networks, data, or packetnetworks, cable networks, satellite networks, private networks, publiclyswitched telephone networks (PSTN), or other types of communicationnetworks. The networks of the communications environment 200 mayrepresent a single communication service provider or multiplecommunications services providers. The features, services, and processesof the illustrative embodiments may be implemented by one or moredevices of the communications environment 200 independently, or as anetworked implementation.

The IP network 208 is a pack-switched internetwork that allows endpointsto communicate. A packet is a formatted unit of data carried by a packetmode or data network. As shown, the remote node 210 may receive a clocksignal from the Stratum clock 204 through network 208. The Stratum clock204 is one embodiment of a highly accurate reference clock that may beutilized to maintain UTC. The Stratum clock 204 may also be an atomic,GPS, or radio clock or other time keeping device suitable formaintaining highly accurate time information.

The switch 212 may be a local exchange, a wire-line switch, or publicexchange using time domain multiplexing to provide telecommunicationsservices to a particular subscriber, or groups of subscribers. Theswitch 212 may be located at a local telephone company's central office,or at a business location serving as a private branch exchange. Theswitch 212 may provide dial-tone, calling features, and additionaldigital and data services to subscribers, such as the telephone 214utilized by the customer 213.

The customer 213 is an example of a user, residence, building, orlocation of a person or group that may utilize any number ofcommunications services. The customer 213 is shown as a residence in theillustrated example, however, the customer 213 may also be an office,business, individual, group of users, or other entity suitably equippedto receive telephone, data, and other communication services. Thecustomer 113 may utilize multiple communications devices and serviceswhich may include the telephone 214. The telephone 214 may be a standarddevice that provides dialing and voice conversation capabilities. Thecustomer 213 may also utilize a client, such as a personal computer,laptop, or PDA. The client may be a personal computer for performing andexecuting programs and instructions, and accessing and communicatingthrough one or more communications networks.

The remote node 210 may utilize the clock signal from the Stratum clock204 to perform synchronous communications with the TDM networks 206 and208. As further described herein, the clock signal may be determinedfrom a packet arrival time of data traffic or through a secondarywavelength communicated to the remote node from the central office 202.

FIG. 3 is a pictorial representation of communications between a centraloffice 302 and a remote node 304 in accordance with an illustrativeembodiment. The central office 302 and the remote node 304 may beparticular implementations of the switches 102, 104, 106, 108, and 110of FIG. 1. FIG. 3 may illustrate an optical layer permittingcommunication between the central office 302 and the remote node 304.

The central office 302 and the remote node 304 may communicate throughthe fiber optic 306. The fiber optic 306 may include one or morestrands. The example of FIG. 3 illustrates a single strand of fiberoptic with channels A 308 and channel B 310. The strands within thefiber optic 306 may utilize one or more wavelengths, lambdas, orfrequencies. The fibers or strands in the fiber optic 306 may bemultimode fibers or single mode fibers.

The central office 302 includes a UTS. The clock signal may bepropagated to the remote node 304 packet to TDM conversion. In oneembodiment, the UTS may be a bits clock linked to a global positioningsystem (GPS) reference source or an atomic clock.

In a first embodiment, the data traffic may be communicated throughchannel A 308. The channel is a fiber or a wavelength utilized tocommunicate signals, data, or other information. In order to recover aclock signal or frequency from a high accuracy clock at the centraloffice 302, the data packets may be sent only on clock ticks. In oneembodiment, the leading edge of a packet or data signal may represent aclock tick. In another embodiment, the end of a packet, middle or otherinformation linked with the arriving packet may indicate the clock tick.

Data packets are not required to be sent on every tick of the clock, butrather only when data packets are available. In one embodiment, astatus, link, clock, update, or synchronization packet may be sent if adata packet has not been transmitted from the central office 302 to theremote node 304 within a time period. For example, if a data packet hasnot been sent for five milliseconds, a data packet may be sent to theremote node 304 to ensure that the clock signal may be recovered andutilized to update or synchronize one or more clocks of the remote node304.

To maintain an accurate time reference, a clock at the remote node 304may be corrected to correspond with the UTS at regular intervals. Theclock may be extracted by one or more Ethernet switches, routers, orsimilar devices utilizing a standard, such as Institute of Electricaland Electronics Engineers (IEEE) 1588 entitled a “Standard for aPrecision Clock Synchronization Protocol for Networked Measurement andControl Systems”.

In a second embodiment, the data traffic sends and receives between thecentral office 302 and the remote node 304 may be transmitted throughchannel A 308. The clock signal may be communicated from the centraloffice 302 to the remote node 304 through channel B 310. As a result,the clock signal or timing is sent out-of-band for receipt by the remotenode 304. The data traffic through channel A 308 may occurnon-synchronously with less processing and manipulations reducing theexpenses and equipment at both the central office 302 and the remotenode 304. As a result, the clock signal may be more easily recovered atthe remote node 304 for use with synchronous Ethernet, pseudo wire,error checking, or disciplining a local clock signal. For example, TDMand synchronous communications, such as synchronous Ethernet, mayrequire a clock signal that is highly accurate or synchronized with oneor more systems, offices, nodes, devices or connections in order toperform communications.

The illustrative embodiments may track the path delay between one ormore elements, such as the central office 302 and the remote node 304.In one example, the path delay may be calculated by measuring the roundtrip delay of a packet between central office 302 and the remote node304. For example, the delay may be 7 milliseconds and as a result, thecentral office 302 and the remote node 304 may determine that there is atransmission delay of 3.5 milliseconds between the devices or points.Processing or time for analysis may also be included in the transmissiondelay. The clocks may be synchronized based on a single delay test,periodically, or frequently based on existing needs. A single port orinterface may communicate the retrieved clock signal to discipline aclock at the remote node 304. The clock may be a central device thatfurther distributes the clock signal to one or more cards, ports, orinternal or external devices. As a result, timing may be moreefficiently communicated between devices utilizing fiber opticconnections.

In one embodiment, the time or tick difference between a reference clockat the central office 302 or other node and the remote node 304 may beutilized to discipline the clock at the remote node 304. For example,the clock signal may be retrieved by analyzing the signal to determinethe signal or packet characteristics, such as a leading edge as measuredbased on one or more thresholds, corresponding to the clock signal. Theclock signal may also be retrieved utilizing standard clocking processesand standards.

FIG. 4 is a block diagram of a remote node in accordance with anillustrative embodiment. The remote node 400 is one embodiment of aEthernet aggregation switch, router, or other intelligent communicationsdevice. The remote node 400 may include fiber optics 401, opticalinterfaces 402 and 404, a clock 406, a data control plane 408, a timingbus 409, port cards 410, a clock slave 412, circuit emulation logic 414,synchronous Ethernet port 416 (sENET), and T1 port 418. The remote node400 may further communicate with customer premise equipment (CPE) 420that may include a clock 422. The remote node 400 is a particularimplementation of the remote node 304 of FIG. 3. In one embodiment, theremote node 400 includes a chassis for receiving a plurality ofexternally connectable elements. The remote node 400 may utilize variousplanes to route data, information, and signals which may include aforwarding plane for communications between ports, the data controlplane 408 that acts as a switching or routing plane for moving datapackages from one location to another, and a timing plane for moving aclock signal.

The remote node 400 may maintain a clock signal disciplined throughsignals communicated by the fiber optics 401 for multiple interfaces,systems, clocks, cards, ports, and downstream devices and elements. Theremote node 400 may further include any number of processors, memories,busses, circuits, sockets, ports, and other elements.

The optical interfaces 402 and 404 are optical transceivers for bothtelecommunication and data communications applications. In oneembodiment, the optical interfaces 402 and 404 are optical Ethernetinterfaces, such as a small formfactor pluggable (SFP). The opticalinterface 402 is a receptacle for interfacing the fiber optics 401 withthe data control plane 408. The optical interfaces 402 and 404 maycommunicate or be connected through a mother board, bus, or otherdevice. The optical interface 404 may interconnect a mother board of theremote node 400 to an network cable including an unshielded twisted pairof a network cable.

In one embodiment, the optical interfaces 402 and 404 may extract theclock signal for communication to the clock in any number of formats.For example, the optical interfaces 402 and 404 may determine the clocksignal utilizing a timing characteristic of a packet, the clock signalmay be transmitted to the clock 406 as digital or analog values,signals, or information. In another example, the optical interfaces 402and 404 may retrieve the clock signal through a wavelength out-of-bandwith data traffic through a modulated signal or data packets includingthe clock signal.

In another embodiment, the optical interface 402 and 404 may communicatethe clock signal to the clock 406 for retrieval or extraction. The clock406 may be disciplined based on arrival timing of an incoming datapacket. In one embodiment, the arrival timing may be a leading edge ofan Ethernet or data packet. The clock 406 may adjust itself up or downas needed. If the clock 406 is correct, no disciplining or adjustmentsare required. The clock 406 may similarly discipline, synchronize, orupdate one or more other clocks, such as the clock slave 412 and theclock 422. The clock signal is also passed to the line, circuitemulation, or port cards 410 of the remote node 400. The port cards 410are a particular representation of an interface to other devices, nodes,connections, or customer equipment.

In one embodiment, the remote node 400 may be equipped with a separatetiming bus 409 from the packet side of the remote node 400 to thetributary or line side of the remote node 400. The timing bus 409 is atransmission path that distributes or transfers the clock signal outsideof the data path bus. The timing bus 409 may be wires, leads, electricalconnections, or other conducting mediums. In one embodiment, the clock406 is an IEEE 1588 clock that communicates a clock signal to the portcards 410 through the timing bus 409. The circuit emulation logic 414,cards utilizing pseudo wire and T1 port 418, and Ethernet port 416 mayutilize the clock signal for TDM, synchronous Ethernet, or other similarservices. The different cards and ports of the remote node 400 may relyon the clock 406 instead of free running or utilizing more expensivealternatives. The port cards 410, circuit emulation logic 414, T1 port418, and the Ethernet port 416 may send synchronous communicationselectrically or optically to any number of other devices. The CPE 420 isterminating equipment at a subscriber's premises that communicatesdirectly or indirectly through a network with the remote node 400. TheCPE 420 may include telephones, modems, terminals, exchanges, switches,routers, set-top boxes, and other similar network equipment. The CPE 420may include the clock 422 that may be further updated or disciplinedbased on communications from the remote node 400.

In one embodiment, the front edge of a data packet may be utilized toindicate a tick of the UTC clock for disciplining in the clock 406. Thesize of data packets may vary and as a result, the leading edge of adata packet may be sent from the central office or another node only ona corresponding clock tick. The central office may send a data packetwithin a specified time period, range, or interval to ensure the clock406 is properly disciplined. The clock 406 may distribute timing to theport cards 410 for replication of TDM and synchronous Ethernet services.In one embodiment, the clock 406 may determine or extract the clocksignal received from the fiber optics 401 before the data in the datapacket is extracted or further communicated. The disciplined clock 406may better allow for error checking and avoiding clock skew.

In another embodiment, the optical interface 402 may extract the clocksignal, timing, or frequency of a UTC clock from a secondary channel.The UTC clock may utilize any number of international or domesticstandards and protocols for maintaining correcting timing. For example,the UTC clock may be a Stratum 3 clock. The optical interface 402 mayinclude a port for connecting to the fiber optics 401. A strand of thefiber optics 401 may communicate utilizing any number of wavelengths. Inone embodiment, a dedicated wavelength is utilized to communicate timingto the remote node 400. As a result, the incoming data packets may besent asynchronously reducing the processing power and analysis requiredat the sending side for communications.

In one embodiment, the incoming clock signal may be modulated utilizingthe second wavelength to indicate clock ticks. In another embodiment,data within data packets sent utilizing the second wavelength mayindicate the time of day, clock tick, or other timing information ordata. The clocking signal is similarly utilized to discipline the clock406. The optical interface may split, filter, or separate thewavelengths. The data packets are sent to the data control plane 408 forrouting and the clock signal is communicated to the clock 406.

FIG. 5 is a flowchart of a process for disciplining a clock of a remotenode in accordance with an illustrative embodiment. The processes ofFIGS. 5 and 6 may be implemented by a remote node. The remote node mayreceive communications through one or more strands of fiber optics. Forexample, the remote node may communicate with a central office utilizinga separate strand for data and telecommunication signals transmitted andreceived. The remote node may include an optical interface forcommunicating through the fiber optics, a clock that may be disciplinedbased on signals received through the fiber optics, port cards, and anynumber of other downstream devices that may be integrated with theremote node or communicate with the remote node.

The process of FIG. 5 may begin by receiving a data packet at the remotenode (step 502). The data packet may include or encapsulate data orother information that is frequently exchanged between nodes in anoptical network. The data packet may be received from a central office,another node, or any number of other devices, systems, networks, orconnections.

Next, the remote node retrieves a clock tick from a leading edge of anincoming data packet (step 504). The data packet sent in step 502 may besent to coordinate with a tick, clock signal, or reading of a UTC,stratum, or reference clock available at another node or at the centraloffice. The sending device may coordinate one or more data packets sothat the leading edge of the data packet always corresponds with a clocktick. The leading edge may correspond or represent the clock tickregardless of whether one or more clock ticks are skipped betweentransmission of data packets. In one embodiment, only the leading edgeof every other data packet or some other multiple may correspond with aclock tick. Utilizing the leading edge of the incoming data packet is aparticular embodiment. Another embodiment may associate a trailing edgeof the data packet with the clock tick. Similarly, any timingcharacteristic of the arrival of the data packet or packetcharacteristic may correspond or be associated with the clock tick.Depending on the type of communication, the clock tick may correspond toa millisecond, microsecond, nanosecond or other portion, multiple, orfactor of a second. During step 504, any number of standards may beutilized by the interface to retrieve the clock signal and thensubsequently for the clock to communicate the clock signal through atiming bus or plane separate from a data bus or plane.

Next, the remote node disciplines a clock in the remote node to theclock tick retrieved from the leading edge of the incoming data packetto generate a clock signal (step 506). The clock in the remote node maybe adjusted by varying the voltage or oscillations of the clock todiscipline and update the clock based on the clock tick.

In one embodiment, synchronous Ethernet standards, such as theInternational Telecommunication Union (ITU) TelecommunicationStandardization Sector (ITU-T) Recommendation G.8261/Y.1361 may beutilized to retrieve and discipline the clock. ITU-T RecommendationG.8261/Y.1361 entitled “Timing and Synchronization Aspects in PacketNetworks” specifies the upper limits of allowable network jitter andwander, the minimum requirements that network equipment at the TDMinterfaces at the boundary of these packet networks may tolerate, andthe minimum requirements for the synchronization function of networkequipment. Other similar standards may be similarly utilized. Duringstep 506, the remote node may also set the intermittent time of day ortime of day setting to coordinate with the clock tick from the UTCclock. In some embodiments, the clock within the remote node may berequired to have a specific degree of accuracy in order to ensure thatthe timing and clock signal tracked or generated by the clock issufficiently accurate to discipline the clock based on incoming datapackets.

Next, the remote node distributes a clock signal to one or more otherclocks (step 508). The other clocks may include subservient or slaveclocks that communicate with the clock of the remote node. For example,a master clock of the remote node may be disciplined based on theincoming data packets. The master clock may then communicate the clocksignal or updated clock signal to one or more servant clocks. A timingbus or plane may communicate the clock signal.

Next, the remote nodes utilizes the clock signal to perform TDM pseudowire (step 510). The other clocks may be integrated or communicate withinterfaces, cards, ports, elements, or other devices that utilize theclock signal to perform TDM or synchronous communications. During step510, the clock signal may be used to emulate a TDM pseudo wire point forefficient and error free communications. The remote node mayadditionally communicate the clock signal to one or more additionalnodes utilizing a timing characteristic or other signaling scheme,standard, or protocol. The process of FIG. 5 may allow the remote nodeto more accurately discipline the clock and distribute the clock signaltiming or frequency to one or more other elements.

FIG. 6 is a flowchart of a process for communicating a clock signal inaccordance with an illustrative embodiment. The process of FIG. 6 maybegin by receiving data traffic through a first channel (step 602). Thefirst channel may represent a wavelength or lambda of a fiber opticstrand. The data traffic may include telecommunications and packetcommunications communicated within an optical network.

Next, the remote node receives a clock signal through a second channel(step 604). The clock signal may be transmitted to the remote node froma UTC clock. The second channel may be a second wavelength communicatedthrough the fiber optic strand the first channel utilizes. The clocksignal may be a wavelength or color designated by the ITU forcommunicating a clock signal, timing or frequency. In one embodiment,the clock signal may be communicated by modulating the light received bythe remote node. For example, the light source may be rapidly turned onand off to represent clock ticks. In another embodiment, the clocksignal may include, data packets that contain information regarding theapplicable clock tick, intermittent time of day, time of day, or othertiming information. The remote node may determine the clock signal byanalyzing the signal or data of the second out-of-band channel.

Next, the remote node disciplines a clock in the remote node to theclock signal retrieved from the second channel (step 606). The clock maybe disciplined by adjusting the time of day, clock ticks, oscillation,frequency, or other information utilized by the clock to track timinginformation. The remote node distributes the clock signal to one or moreother clocks (step 608). The remote node may include one or more slaveclocks that receive the clock signal through a timing bus that operatesindependent of the data bus. The slave clocks may be in the remote nodeor one or more other interfaces, cards, or downstream devices. The clocksignal may be utilized to perform TDM pseudo wire and any number ofsynchronous communications.

Next, the remote node transmits the clock signal to a next node (step610). The clock signal may be communicated utilizing fiber optic orelectrical connections to the next node. In one embodiment, the clocksignal may be communicated utilizing a second channel similar oridentical to the way the clock signal was received in step 604. Anynumber of other communications standards and methods may also beutilized including the process described in FIG. 5. The next node may bea switch in a ring, CPE, or other device, equipment or system.

In one embodiment, the remote node may be configured to determinewhether the clock signal is being received in step 604. If the clocksignal, not being received through the second channel, is incorrect, orhas faltered, the remote node may utilize logic or an algorithm toperform clock discipline in a secondary clock signal. For example, asecondary clock signal may be incoming from another side of a metroEthernet ring of which the remote node is part of

The previous detailed description is of a small number of embodimentsfor implementing the invention and is not intended to be limiting inscope. The following claims set forth a number of the embodiments of theinvention disclosed with greater particularity.

What is claimed:
 1. A method for synchronizing a clock for datatransmissions, the method comprising: receiving a data packet at aremote node, a timing characteristic of the data packet corresponding toa tick of a clock from a reference clock; determining the tick of theclock based on the timing characteristic of the data packet;disciplining a secondary clock with the reference clock by adjusting thesecondary clock based on a difference between times measured by thereference clock and the secondary clock to generate a clock signal;generating the clock signal regardless of whether the data packet isreceived for disciplining the secondary clock; and communicating theclock signal to one or more interfaces.
 2. The method according to claim1, wherein the timing characteristic is a leading edge of the datapacket.
 3. The method according to claim 1, wherein one or more datapackets including the data packet utilized for determining the tick ofthe clock are not received for each and every tick of the clockgenerated by the reference clock.
 4. The method according to claim 1,wherein the data packet is an Ethernet packet received from a centraloffice through a metro Ethernet network, and wherein the reference clockis a Stratum clock.
 5. The method according to claim 1, wherein thesecondary clock is adjusted to an accuracy of at least one millisecond.6. The method according to claim 1, further comprising: determining adelay between the reference clock and the remote node, wherein thedisciplining is performed utilizing the delay.
 7. The method accordingto claim 1, sending an inactivity packet at an interval in order tosynchronize the secondary clock.
 8. The method according to claim 1,further comprising: determining the tick of the clock before separatingdata from the data packet.
 9. The method according to claim 1, furthercomprising: communicating the clock signal from the secondary clock toone or more slave clocks; and performing error checking within theremote node utilizing the clock signal.
 10. A remote node forcommunication with a synchronous network, the remote node comprising: anoptical interface for receiving a data signal through a fiber optic, theoptical interface operable to extract a tick of a reference clock from adata packet included in the data signal, the tick of the reference clockbeing associated with a leading edge of the data packet, wherein thereference clock is an atomic clock; a clock in communication with theoptical interface, the optical interface disciplininges the clockutilizing the extracted tick of the reference clock to maintain a clocksignal, wherein the clock tracks time using the leading edge and withoutbeing updated by the reference clock; a timing bus in communication withthe clock, the timing bus operable to transmit the clock signal to oneor more cards; and one or more interfaces in communication with thetiming bus, the one or more interfaces operable to utilize the clocksignal for synchronous communications.
 11. The remote node according toclaim 10, wherein the remote node is a switch, wherein the one or moreinterfaces are one or more cards, and wherein the timing characteristicis a leading edge of the data packet.
 12. The remote node according toclaim 10, further comprising: a slave clock in communication with thetiming bus, the slave clock utilizes the clock signal for thesynchronous communications through the one or more interfaces.
 13. Theremote node according to claim 10, wherein the interface disciplines theclock utilizing a propagation delay between sending node and the remotenode.
 14. A method for synchronizing a clock for data transmissions, themethod comprising: determining a propagation delay between a sendingnode and a remote node; receiving a data packet at the remote node inresponse to a first data stream being available, a leading edge of thedata packet corresponding to a tick of a clock from a reference clock;determining the tick of the clock from the leading edge of the datapacket; disciplining a secondary clock with the reference clock byadjusting the secondary clock based on a difference between timesmeasured by the reference clock and the secondary clock from the tick ofthe clock and the propagation delay to generate a clock signal inresponse to receiving the first data stream, wherein the secondary clockis disciplined based on data packets received through a secondary datastream in response to determining the first data stream including thedata packet is unavailable; and communicating the clock signal to one ormore interfaces.
 15. The method according to claim 14, wherein theleading edge of a plurality of data packets received by the remote nodecorresponds to the tick of the clock, wherein the reference clock is anatomic clock, and wherein the clock tracks time when data packets arenot received.
 16. The method according to claim 14, further comprising:determining the clock signal from another data stream in response to notreceiving the data packet; disciplining a slave clock in communicationwith the secondary clock through a timing bus, the slave clock beingutilized by the one or more interfaces.
 17. The method according toclaim 14, wherein the communicating is performed through a timing bus,wherein the remote node is a metro Ethernet switch, wherein at least oneof the one or more interfaces perform synchronous communicationsrequiring the clock signal.